The spec requires synapse to support `identifier` dicts for `m.login.password`
user-interactive auth, which it did not (instead, it required an undocumented
`user` parameter.)
To fix this properly, we need to pull the code that interprets `identifier`
into `AuthHandler.validate_login` so that it can be called from the UIA code.
Fixes#5665.
By persisting the user interactive authentication sessions to the database, this fixes
situations where a user hits different works throughout their auth session and also
allows sessions to persist through restarts of Synapse.
Pull the checkers out to their own classes, rather than having them lost in a
massive 1000-line class which does everything.
This is also preparation for some more intelligent advertising of flows, as per #6100
